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I am actually a pretty good electrician, but am baffled by what I found today.

During a remodel and adding a few new boxes for future lights, I received a shock from a neutral wire and a floating ground.

The house is old and there are no grounds on most of the circuits. I was confused for there is no ground except for about 20 ft going from one new box to another but not connected to panel ground. In fact the ground is terminated with a wire nut in each box but are not connected to anything. Yet when I tested the neutral to ground voltage, I found 48 volts.

Subsequently I tested a few other outlets in the house and also found neutral to ground voltage of 48 v. I then unplugged all devices. Voltage to ground disappeared. Only when I plugged in a device with a ground did the neutral to ground voltage reappear. I then went to the main panel thinking I had a bad neutral somewhere, and disconnected each circuit one at a time and found no neutral to ground issues at the panel and the neutral voltage only disappeared when I turned off hot to circuit.

Different circuits were found to have neutral to ground voltage only when a grounded plug was put into that outlet. I assumed that the ground of the device was allowing me to read neutral voltage. Hot to neutral is correct everywhere. I am baffled. Suggestions?

  • What do you get when you measure hot to ground? – Tester101 Jul 8 '14 at 2:42
  • Did you test the device for ground faults? – Tester101 Jul 8 '14 at 2:48
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    Floating ground? As in you have ground wires that are not bonded to the neutral at the panel? That's a recipe for disaster. – longneck Jul 8 '14 at 3:51
  • Is there any other metallic connection between boxes, like conduit or that old silverish romex? – wallyk Jul 8 '14 at 7:13
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    @Greebo: try reading a book. It will get you used to walls of text. – cdonner Jul 8 '14 at 21:30
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The AC input circuits of some devices use a power line filter circuit that works to keep high frequency stuff inside the equipment and prevent fast spikes from entering the product from the power lines.

The schematic of a typical power line filter looks like this.

enter image description here

The two capacitors that you see in the filter from line side to earth and from the neutral side to earth can form a voltage divider for AC voltage and place approximately half the line voltage on the earth wire. This would be the case for instances where the earth ground connection was open circuit.

Normally this is not a shock hazard issue because the neutral wire is shorted to the earth ground connection at the power panel.

The fact that you found this AC signal present demonstrates that some part of your house wiring has a open earth ground wire someplace. For the safety of your self and your family it is essential to get this fault corrected. The floating earth wire is an invite to it not performing its safety function when you least expect to need that safety feature.

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If there's some resistance in the neutral path, there will be voltage across it when there's current running through it. Presuming that your neutral runs to earth, I'd suggest checking that this connection is still good, and that nobody did something stupid like connecting to earth through the water main and then replacing part of that line with plastic.

Aluminum wire is notorious for bad connections, if you have any of that in the place.

The other possiblity is that there's leakage to your floating ground. Which is exactly why you shouldn't have a floating ground; the purpose of safety ground is to cause the bad circuit to blow a breaker before it shocks you or catches fire.

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If your neutral floats, it will drift somewhat proportionally to the difference of the load in both legs and the internal resistance of the source.

Are both legs of a home's power supply equally used?

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